Adsorption and desorption of simazine on a Hiroshima loam soil and its colloidal size fractions were studied in batch experiments. The colloidal size fractions were isolated by repeated gravity sedimentation. Two-step sorption kinetics (step I-with initially non-polluted soil and step II-with soil initially polluted during step I) were well described by a modified Freundlich type kinetics sorption model. Both for the whole soil and soil size fractions the sorption kinetics were characterized by a rapid initial adsorption where the major part of the pesticide was adsorbed within the first 10 minutes followed by a slow, diffusion-controlled sorption. The sorption kinetics parameter N was well correlated with soil organic matter content, and N decreased from step I to step II, dependent on the total amount of simazine adsorbed during step I. This suggests that adsorption kinetics were controlled by both the actual organic matter content and the previous sorption history of each soil size fraction. Adsorption and desorption isotherms for whole soil and loam soil size fractions were well described by the classical Freundlich isotherm equation. A fine silt size fraction with a relatively higher soil organic matter content adsorbed the most simazine and, also, released the least simazine after 10 steps desorption. A large difference between the adsorption and desorption isotherms and therefore a hysteretic sorption behavior for all soil size fractions was observed. Significant correlation was found between adsorption and desorption coefficients and the soil organic matter content.